As mentioned in Patent Documents 1 to 3 for example, common photosensitive resin plates for flexography are manufactured via exposing, developing and post-exposing steps. A printing method using the photosensitive resin plate for flexography is such a method where ink is supplied onto the convex surface of uneven resin plate using an ink-supplying roll or the like and then the resin plate is made to contact with the thing to be printed so that the ink on the convex surface is transferred to the thing to be printed. In the flexography as such, adhesion of the ink to a shoulder area of the convex surface of the resin plate or invasion of the ink to the concave surface (hereinafter, it will be referred to “plate surface contamination”) often happens during the printing for long time. As a result, even the area which is not the inherent pattern area may be printed resulting in deterioration of the printing quality.
Particularly in recent years, application of flexography to a highly precise printing has been in progress and a halftone dot printing in high line numbers has been briskly carried out for creating the gradation of color and concentration. Since intervals among the halftone dots become smaller in that case, the ink is more apt to be filled in the concaves of the halftone dots of the printing plate. When the ink is accumulated in the concaves of the printing plate in a predetermined amount or more, it transfers to the thing to be printed to generate the dot bridging resulting in a significant deterioration of the printing quality.
One of the characteristics of flexography is that it can cope with various types of ink such as aqueous ink, alcoholic ink, solvent-type ink or UV ink. On the other hand, there has been a demand for preventing plate surface contamination even when any type of ink is used. It is particularly necessary to prevent plate surface contamination even if an ink having high permeability to the plate material such as solvent-type ink or UV ink is used.
Several methods have been proposed already concerning the prevention of plate surface contamination of a flexographic printing plate. For example, Patent Document 4 discloses a technique where silicone oil, silicone rubber, silicone varnish or fluorine compound is adhered, by means of brushing or spraying, onto a printing plate mainly comprising polyester resin. Although some preventing effects for plate surface contamination are available by such a technique, the plate surface contamination cannot be prevented in a halftone dot printing in high line numbers having narrow intervals among the dots. Persisting effect is also insufficient.
In Patent Document 5, there is proposed a method where a mixture of an aqueous resin and an aqueous emulsion of silicone compound or fluorine compound is applied onto a printing plate comprising an elastomer. However, due to the use of a non-modified silicone compound, an effect of preventing the plate surface contamination is not sufficient. Especially in a long-run or repeated printing, said effect hardly continues and it is necessary to apply the mixture repeatedly for maintaining the effect.
In Patent Document 6, there is proposed a method for contacting a flexographic printing plate comprising hydrophilic polymer, styrene-butadiene-styrene block copolymer, liquid butadiene, photo-cross-linking agent and photoinitiator with a solution containing a modified silicone compound. In said invention, an improvement in the effect of preventing the plate surface contamination is noted due to the use of a modified silicone compound. However, in a long-run printing, plate surface contamination is generated and, particularly when a solvent-type ink and UV ink are used, its persisting effect is not sufficient. In said invention, there is also mentioned a method where active ray is irradiated after contacting a modified silicone compound with the plate but the persisting effect is not sufficient in such a case as well.
In Patent Document 7, there is disclosed a method where an organosilicon compound is made to contain in a printing plate comprising block copolymer, photo-cross-linking agent and photoinitiator. In that method, silicone is made to contain in a resin composition and an excellent persisting effect is achieved as compared with a method where silicone solution is contacted with a plate surface. However, the organosilicon compound is spread into the entire printing plate composition and amount of silicone existing on the printing plate surface is small. Therefore, the effect of preventing the clogging of ink which is crucial is not sufficient. Deterioration of adhesion of resin layer to base film is also resulted and a serious disadvantage of detachment of base film and resin layer during the printing may happen.